1. Field of the Invention
The present invention relates to the field of integrated circuits devoted at least in part to the processing of analog signals of relatively low frequencies, such as voice signals.
2. Prior Art
The preferred embodiment of the present invention is intended for use in analog signal storage and playback devices of the type manufactured by Information Storage Devices, Inc. of San Jose, Calif. These analog storage and playback devices are single integrated circuits for receiving an analog signal in the audio range, typically from a microphone or other audio signal input device, and storing successive samples of the audio signal during record, and for later providing an output signal, typically to a speaker, representing the successive playback of the stored audio signal samples. In that regard, as used herein, the phrase "audio signal" is used generally to refer to electronic signals in a typical voice frequency band, but includes other types of such signals, such as analog signals representing music, sound effects, alarms, or other signals having similar low frequency cut-off. In that regard, the present invention is directed toward problems associated with integrating analog processing circuits having relatively low, low frequency cut-off using all on-chip components.
FIG. 1 is a simplified block diagram illustrating typical audio signal processing in a system wherein the analog signal is to be periodically sampled and stored in non-volatile memory for later playback, in the case of the Information Storage Devices, Inc. products, by storage of the analog signal sample itself, and in other storage systems by digitizing the analog sample and storing the digitized value of the sample in digital form. In either case, an input device 20 such as a microphone is used to provide an input to a preamplifier 22, the output of which is capacitively coupled through capacitor C1 to amplifier 24. The output of the amplifier 24 controls an automatic gain control circuit 26 to adjust the gain of the preamplifier 22 to hold the output of amplifier 24 substantially constant, independent of the amplitude of the signal provided to the preamplifier. Because of the subsequent sampling, an anti-aliasing filter 28, typically an analog anti-aliasing filter, is coupled to the output of amplifier 24 to limit the upper end of the bandpass of the analog signal consistent with the sampling rate used. In the prior art double-ended preamplifier, amplifier and anti-aliasing circuits were used.
Since the typical voice frequency range runs from approximately 100 hertz to a few kilohertz, a large RC time constant is required to implement the appropriate low frequency pole in the frequency response of the audio signal path. By way of example, a pole at 150 hertz would require an RC time constant of 1 millisecond. To achieve this time constant, a realistic monolithic resistor of 100K ohms would require a capacitor of 10.sup.4 picofarads. Alternatively, a realistic monolithic capacitor of 10 picofarads would require a resistor of 100 Mohms. Therefore, in the prior art, the AC coupling is usually done with an on-chip resistor, but with an external capacitor. The gain of the amplifier is usually controlled by a resistance ratio using on-chip resistors.
FIG. 2 illustrates the typical connection of the amplifier. The capacitor C1 is an off-chip capacitor having a value on the order of 1 .mu.f, with the on-chip resistors R1 and R2 having values in the order of tens of Kohms. The capacitor C1 and input resistor R1 define the low frequency pole, and the resistor ratio R2/R1 defines the gain of the amplifier.
The prior art circuit just described is both simple and functional. However, it also has certain disadvantages not limited to, but particularly important in, very low cost and/or miniature systems. One disadvantage, of course, is the size and expense of the external capacitor, which in turn increases size and cost of the printed circuit board or whatever other circuit packaging technique is used, and of course also increases the size of the final packaged system. Further, the connection between the preamplifier and amplifier of FIG. 1 need not be available to the outside world except for the use of the external capacitor, and accordingly, the prior art technique of using the external capacitor requires two more bonding pads on each integrated circuit die, increasing the size of the die and perhaps the integrated circuit package, depending on how the same is packaged. Consequently, overall cost and size considerations make the use of the external capacitor undesirable, particularly in low cost and/or miniature systems. Further, while one can reasonably control the ratio of resistors obtained during integrated circuit fabrication, the absolute magnitude of a particular resistance is not well controllable. Thus, the low frequency pole is not well controlled using an external capacitor, as the pole is determined by the combination of the external capacitor and an internal resistor value. The internal resistor itself can vary as much as 30 percent, to which the tolerance of the external capacitor must be added. Thus, it would be highly desirable to have both the coupling capacitor and the associated resistor on-chip to avoid the requirement of bringing out the two additional connections from the integrated circuit and the increased size and expense resulting from the inclusion of the external capacitor.
It is also known in the prior art that a capacitor may be switched to simulate a resistor. A typical switched capacitor circuit is shown in FIG. 3, wherein capacitor C has one end connected to a fixed voltage such as ground, and the other end connected to two switches, SW and SW operating in a non-overlapping complementary manner so that both switches are not closed at the same time.
In this circuit, when switch SW is closed, capacitor C adjusts its charge so that q.sub.1 =CV.sub.1. When switch SW opens and then switch SW closes, the charge on the capacitor will readjust through the output V.sub.2 to q.sub.2 =CV.sub.2. Thus the charge delivered to the output V.sub.2 from the input V.sub.1 is equal to q.sub.1 -q.sub.2 =C(V.sub.1 -V.sub.2). If this is repeated N times per second, then the charge transfer per second, or current i, will be dq/dt=i=NC (V.sub.1 -V.sub.2). Thus on average, the capacitor looks like a resistor between V.sub.1 and V.sub.2, passing a current proportional to the voltage difference V.sub.1 -V.sub.2 with an apparent resistance equal to 1/NC.